Flywheel energy storage (FES) system is a way of energy storage, mainly by accelerating the rotor (having a flywheel) to a very high speed, such that energy can be stored in the system as rotational kinetic energy. When the system needs to output energy, according to the principle of conservation of energy, the rotational speed of the flywheel decreases; when the system needs to store energy, it can increase the rotational speed of the flywheel.
In detail, a typical flywheel energy storage system consists of a chamber containing a flywheel and an electric motor assembly assembled to the flywheel. The flywheel has mechanical potential energy while rotating. The stored energy grows in proportion to the mass and the rotational speed of the flywheel. And when the flywheel's torque with respect to the shaft increases, the stored energy increase in proportional to torque squared. The electric motor assembly acts as an energy output/input device, and it can receive the electric power in the form of a motor to drive the flywheel to rotate, and also can convert the mechanical potential energy of the flywheel into and electric power in the form of a power generator.
Therefore, the flywheel energy storage system can operate as an electric motor or a power generator to directly convert mechanical energy and electrical power. Since the rotational speed of the flywheel can be quickly boosted to absorb or release energy, its power is greater than other energy storage device. The power density of the flywheel energy storage system is significantly higher than that of chemical batteries (e.g., lead-acid battery), making the flywheel energy storage system more suitable than the chemical batteries in some applications that demands fast energy storage. As such, the flywheel energy storage system is much widely used in some applications, such as vehicles or other power generating system suchlike household wind power system or independent mobile energy storage unit similar to the chemical battery. The above reasons and advantages make the flywheel energy storage system become more important than ever.
In order to improve the performance of the flywheel energy storage system, the air resistance to the internal components has to be decreased. One of the efficient way is to enclose all of the internal components in the casing and then vacuum the casing to create a vacuum environment for these components. Meanwhile, the motor stator of the electric motor assembly needs to connect to the external device via cable for receiving or outputting electricity; thus, the casing has to have a through hole for the cable. Although the through hole will be sealed after the cable has been disposed through, it still can find vacuum leak at the through hole and thus negatively affect the airtightness of the system. As a result, the performance still fails to meet the requirement. In addition, the vacuum internal space and the outer environment have a very large pressure difference, which would aggravate the vacuum leak. To solve such problem, some would constantly vacuum the system to maintain the vacuum condition, but it is energy wasting and increasing cost.
Further, in order to dissipate heat generated while the motor stator is in operation so as to prevent the accumulation of excessive heat in the coil of the motor stator to cause the winding to burn out or cause the motor to shut down. Moreover, the casing also needs to form another through hole for the cooling pipe. It is understood that the another through hole increases the possibility of air entering into the system, making the airtightness in the system more difficult to maintain
As such, how to maintain the vacuum condition of the flywheel energy storage system to solve the aforementioned problems becomes an important topic in the field.